Disk drives typically include a data storage disk which is rotated relative to a base at a controlled velocity. A fixed data transducer head assembly is typically radially positionable relative to concentric data tracks formed on a storage surface of the disk. It is common to find disk drives with a single disk which may be removable, as in the case of a fixed, floppy or cartridge hard disk drive or multiple disks mounted to a common spindle as is commonly found in fixed disk drives. With disk drives employing positionable heads, it is axiomatic that a control mechanism must be employed in order to move the head from a departure track to a destination track during track seeking operations, and to maintain the head in registration with a track during track following operations when user data is being written to the disk or read from the disk.
Head position control systems for disk drives have followed many forms. One form employed in low track density disk drives has been a so-called open loop servo positioner employing a detent-providing stepper motor. In order to access a particular track location, a controller issues step pulse signals to the step motor and its armature rotates accordingly, with a unit increment of rotation occurring with each step pulse control signal. This incremental rotation is then applied to rotate a rotary head positioner, or it is converted into rectilinear motion for moving a linear head positioner. Since the step motor remains at a stable position between steps, this characteristic has been relied upon to ensure stability of the head positioner during track following. Open loop servo positioners have most frequently been employed in floppy disk drives, and have also been employed in some low cost, limited capacity fixed disk drives, such as the Shugart Associates' SA1000 eight inch fixed disk drive, and the Seagate Technology ST-506 and ST-412 five and one quarter inch disk drives. A three and one half inch disk diameter, stepper motor positioned disk drive is described in U.S. Pat. No. 5,568,988, for example.
A drawback of the open loop servo head positioner is that without any head position feedback information, the concentric data tracks must be spaced sufficiently apart in order to accommodate tolerances, such as those resulting from thermal expansion and contraction of mechanical components, occurring within the disk drive during operation.
A second approach has been to dedicate an entire data storage surface of a disk drive to head position servo information. With this approach, a pattern of concentric servo tracks is written very precisely with a servo writer apparatus. The disk drive is then Equipped with a servo head which is mounted for common movement with the data transducer heads by a head arm assembly. A servo read-only channel connected with the servo head operates within a closed loop head position servo system. These systems have been primarily analog in nature. During track seeking and track following, phase coherent analog servo patterns recorded within the servo tracks are read by the servo head and thereby provide actual position information used by the closed loop servo to correct the head position to either a desired trajectory during seeking or to track centerline alignment during following.
One drawback of the servo surface approach is that an entire data storage surface of the disk drive is required for servo information overhead and is therefore unavailable for the primary task of storing user information. Also a separate servo head and read channel are required with this implementation. While track densities can be very high, the servo surface approach, typically employing multiple disks on the spindle, is generally viewed as a high cost approach. One other potential drawback is that over thermal operating cycles or after mechanical shocks, a positional discrepancy may develop between data recorded on a track of a disk other than the disk containing the servo pattern and the corresponding servo track provided for nominal centerline registration of the stack of servo and data transducers forming the single head arm assembly. Yet another drawback with analog servo systems is their susceptibility to noise and other interferences, which may limit their performance, or which may limit yields in mass production.
A head position system which may be realized at lower cost than the dedicated closed loop servo but which does not require the cost overhead of the dedicated servo surface and dedicated servo transducer and servo read channel, is realized with a head positioner transducer, such as a polyphase optical encoder having a scale tightly coupled to the head arm assembly. The heads are then positioned on the basis of position information fed back to the servo control loop from the position transducer. Unfortunately, system tolerances and shifts, typically due to thermal changes, inertia, reticle to scale gap shifts, etc., cause the optical encoder to lose calibration with actual head position.
One way of correcting for tolerances of the disk drive arising e.g. from thermal shifts, or otherwise, is to embed prerecorded servo information on one or more of the data storage surfaces, and to retrieve this embedded servo information periodically and use it as a position correction vernier in order to correct the position of the head transducers relative to the data track location. This correction information may be embedded as a single servo sector located at a raw index marker, as was done in the commonly assigned U.S. Pat. No. 4,396,959, now U.S. Reissue Pat. No. RE32,075 in the case of the polyphase optical encoder positioner based servo control loop. Or, the information may be embedded as one or more servo sectors and used in combination with a servo surface, as was done by IBM in its 62 PC eight inch disk drive known as the "Piccolo". see Robert D. Commander et al., "Servo Design for an Eight-Inch Disk File", IBM Disk Storage Technology. February 1980, pp. 90-98; also see IBM U.S. Pat. No. 4,072,990 relating to this general approach.
Still another method for providing head position feedback information to a head positioner servo loop is to embed servo information in sufficient quantity and readable at sufficiently frequent intervals within a data storage area to provide for head position control. Such information may be of the form of dedicated tracks interspersed between data tracks, or more frequently, such information may be of the form of servo sectors which interrupt the data tracks. With this approach, the servo information is periodically sampled and held, and head position is derived from the samples. In an embedded servo sector scheme, head position resolution will depend e.g. upon the number of samples provided per revolution of the disk and the efficiency with which the servo loop can recover and process each servo sample into a head position correction value for controlling the actuator and thereby correct the position of the head arm and heads.
An exemplary disk drive employing an embedded sector digital head positioner servo loop is described in commonly assigned U.S. Pat. No. 4,669,004, the disclosure of which is hereby incorporated by reference. While the disk drive described in the '004 patent worked very well, certain improvements have subsequently been made; and, this patent represents an improvement in one aspect of the digital servo system employed in the '004 patent.
To be fully effective for its assigned task of providing actual position information, an embedded servo pattern should include information identifying the track as unique from its neighboring tracks, and the pattern should provide a centerline reference as well. The track identification number is useful during track seeking operations to indicate the radial position of the data head relative to the storage surface. To be reliable the track identification number must be in the form of full track addresses, and the track identification number must be readable not only when the data head is following the track during track following, but when the data track is not centered over the servo information but is located somewhere between the tracks, as frequently occurs during track seeking. Embedded servo sectors typically include not only track identification numbers used by the positioning servo for coarse positioning, but also fine positioning information, typically two or four radially offset servo bursts, the relative amplitudes of which are sampled and held. When servo information is embedded within data track zones having differing data transfer rates, issues relating to automatic gain control of the read channel also arise, as it is necessary to reset or normalize read channel gain when traversing a servo sector, particularly when sampling and holding fine positioning or track centering information.
One known technique to improve the robustness and reliability of the track identification number is to encode it as a Gray code. With Gray code addressing, only one bit changes as each track is progressively encountered. Thus, if a data head is located between two tracks, Gray coding of the track number will limit the track error to no more than about one half track pitch. Read channel hysteresis may create a preference for one track over another adjacent track location, such that change of track number may not occur at the equidistant boundary between the two tracks.
The problem associated with reliability of the track identification number is fairly well understood and solved in the case of disk drives practicing invariant data transfer rates, as was the case in the referenced '004 patent. However, it is also well understood that fixed data rates lead to inefficient storage of user data, since flux transition density is limited to the maximum density available at the innermost useable data track location, where relative head-disk velocity is lowest. To solve the problem of non-optimization of data transfer, variable data rates have been proposed. One known approach is to divide the storage surface into a plurality of data zones. Each zone has a data transfer rate established as the practical maximum for the radially innermost track of the zone.
When data zones are present, complications arise with regard to embedding servo information. During longer track seeking operations, the data head will change one or more zones. If the data rate of sector servo information embedded within the data tracks changes with the change in zones, a particular problem arises in decoding track identification numbers, as the servo must resynchronize virtually immediately to a new data rate associated with a particular zone. Another issue presented with zoned data transfer rates is that fixed block length data sectors will differ in circumferential length from zone to zone. If servo information is embedded at the beginning of such blocks, the servo information will not align throughout the disk surface, leading to complications in timing and sampling of the servo information. An example of a zoned data recording scheme wherein the location of embedded servo information varies with zone is to be found in the Ottesen U.S. Pat. No. 4,016,603, the disclosure of which is hereby incorporated by reference.
A hitherto unsolved need has arisen for a servo data recovery circuit for recovering the embedded servo information in a manner which is improved over the prior approaches and therefor more effective for its tasks.